1. Field of the Invention
The present invention relates to an image sensing apparatus, exposure control method, and recording medium, and particularly to an exposure control technique in the image sensing apparatus.
2. Description of the Related Art
An image sensing apparatus such as a digital camera performs image sensing by using exposure settings decided in accordance with an image sensing condition (scene) and a subject. In general, the image sensing apparatus uses the luminance value of a main subject as a reference and decides, as exposure settings, exposure control values such as the shutter speed, aperture value, and sensitivity for obtaining a correct exposure.
One scene where it is difficult to set exposure is a backlight scene. In the backlight scene, the luminance of a main subject is low and that of the background is high. At this time, if exposure settings are decided to correctly expose a main subject, and then an image is sensed, the main subject serving as a dark portion is sensed without blocked up shadows, but blown out highlights may occur in the background. To prevent this, there is an image sensing apparatus capable of outputting an image in which subjects are correctly exposed as a whole in an image sensing condition in which bright and dark portions exist in the image sensing range, like a backlight scene.
Japanese Patent No. 4054263 discloses a technique of combining two types of images obtained at different exposures by using an image sensor including light-receiving elements of different sensitivities, thereby outputting an image in which both bright and dark portions are sensed at correct exposures. Japanese Patent No. 3395770 discloses a technique of sensing a plurality of images at different exposure amounts by time division, and combining them, thereby outputting a tone expression-enhanced image.
To output a tone expression-enhanced image by combining images obtained at different exposure settings, as in Japanese Patent Nos. 4054263 and 3395770, it is necessary to appropriately make exposure settings for the respective images to be combined. The exposure settings are preferably correct for a main subject. For example, in a backlight scene, the exposure settings of a high-exposure image having a large exposure amount are decided to correctly expose a main subject present at a dark portion.
However, the main subject may be erroneously recognized owing to the composition or the like in image sensing, and a subject of the user's choice, which should be selected as a main subject, may not be correctly exposed in an image sensing result obtained at decided exposure settings. For example, in a backlight scene, an image in which an original main subject suffers blocked up shadows may be obtained at exposure settings decided when a subject brighter than the original main subject among subjects present at a dark portion is recognized as a main subject.